How to calculate a centrifugal pump for a jet boat.

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Discussion Overview

The discussion revolves around the design and calculation of a centrifugal pump for a jet boat project. Participants explore the necessary dimensions and specifications for the impeller to optimize performance without overloading the engine, considering factors such as horsepower, torque, and fluid dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant outlines the specifications of their jet boat project, including engine power and torque, and seeks advice on impeller dimensions and RPM to maximize performance.
  • Another participant suggests that optimizing the propeller may involve subjective experience, indicating a lack of straightforward scientific methods.
  • A different participant expresses the need for a formula related to fluid dynamics to achieve maximum thrust with the given horsepower.
  • One reply recommends purchasing a pre-made impeller, emphasizing the complexity of the calculations involved and the potential need for advanced computational tools.
  • A participant shares a resource link to a book discussing the basic principles of waterjet propulsion, potentially offering additional insights into the topic.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to impeller design. There are competing views on whether to pursue custom design or to opt for a pre-made solution, and the discussion reflects uncertainty about the calculations involved.

Contextual Notes

The discussion highlights limitations related to the complexity of fluid dynamics calculations and the potential need for advanced tools, which are not universally accessible to all participants.

beef supreme
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I am building a jet boat as a project this year. I am going construct the impeller my self but don't know what dimensions I need to make it to get the most out of the engines without over loading them. Here's what I have to work with:

The power source is going to be approximately 90 hp at 3600 rpm with about 130 ft-lb of torque. That power is going to be transmitted by two B section v-belts and pulleys. What I'm trying to accomplish is to get as close to 90 hp out of a water jet as possible. I can do about 50 square inches at the input and output of the impeller and a nozzle of about 12.5 square inches at the rear of the boat. My question is what diameter do I want to make the impeller assuming the circumference times the height of the opening at the edge is equal to the intake (50 inches^2) and what rpm?

Any insight on the matter would be greatly appreciated. Thanks
 
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I think you need to go find a jet boat forum. I suspect optimising the prop to match the boat is as much a black art as a science.
 
I really need someone who has a good general knowledge of fluid dynamics and such who can perhaps give me a formula to figure how to achieve the maximum gross thrust with the 90ish horsepower input.
People on boat forums, in my experiences, generally haven't known much about the physics surrounding these things.
 
Honestly, I think you'd be best off buying a premade impeller designed for about your level of power input. This is far from a trivial calculation, and as CWatters said, is probably as much a black art as it is a science (unless you have access to an advanced CFD program and a supercomputer, as well as a fair amount of time to figure out how to run it).
 

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